Throughout this specification the use of the word “inventor” in singular form may be taken as reference to one (singular) or all (plural) inventors of the present invention.
Fire protection and suppressant systems may operate by detecting the presence of smoke and other airborne pollutants or, in general, particles. Upon a threshold level of smoke being detected, an alarm may be activated and operation of a fire suppressant system may be initiated. While the fire itself will cause damage, considerable property damage and also environmental damage may also be caused by operation of the fire suppression system and subsequent removal of the suppressant may be quite hazardous. A detection system, which is sufficiently sensitive to detect an abnormal condition prior to the onset of a fire, is very advantageous as it enables action to be taken at a very early stage before the onset of actual fire conditions. For example, when most substances are heated, even before heating occurs to a point at which a fire commences, emissions will be generated and if these can be detected by a suitably sensitive system, a warning provided at that very early stage may allow the problem to be detected and rectified, or equipment turned off for example, before the fire actually starts.
Aspirated smoke detection systems may incorporate a sampling pipe network consisting of one or more sampling pipes with sampling holes installed at positions where smoke or pre-fire emissions may be collected. Air is drawn in through the sampling holes and along the pipe by means of an aspirator or fan and is directed through a detector at a remote location. Although there are a number of different types of smoke detectors which may be used in a system as outlined above. Optical scatter detectors, have been found to provide suitable sensitivity at reasonable cost. Optical scatter detectors operate on the principle that smoke particles or other airborne pollutants when introduced into a detection chamber and subjected to a high intensity light beam will cause light to scatter. A light sensor senses the scattered light. The greater the amount of smoke particles within the sample introduced into the detector chamber the greater will be the amount of light scatter. The scatter detector detects the amount of scattered light and hence is able to provide an output signal indicative of the amount of smoke particles or other pollutant particles within the sample flow.
A difficulty arises in operation of aspirated smoke detector systems of the above kind in that most atmospheres where smoke or fire detection is required contain dust which may interfere with operation of the system. A filter may therefore be incorporated into the system for the purpose of keeping dust away from sensitive optical surfaces and to prevent dust from artificially affecting the detection of particles indicative of the presence of fire and/or smoke. For example with optical scatter type detectors, the presence of dust may seriously affect the detector output since dust particles may be generally larger than the particles which are indicative of the presence of smoke or fire and produce more scattered light than those particles.
Over time a filter used to reduce dust transmittance into the detection chamber will eventually fill with dust, which may prevent passage therethrough of not only dust particles, but also smoke particles. This causes the effective sensitivity of the detector system to drop. Attempts have been made in the prior art to alleviate this difficulty, for example, see granted U.S. Pat. No. 6,052,058 entitled Filter Integrity Monitoring System in the name of the present applicant, the contents of which are incorporated herein by reference.
When detecting smoke in a given environment, it is important to discriminate between smoke particles and dust particles, both of which will scatter light once in the detection chamber. Ideally dust particles are removed from the airflow path and smoke particles are allowed to continue through to the detection chamber. Thus, in an ideal situation, dust transmittance through the filter would be zero, and smoke transmittance would be 100%. Unfortunately no known filter has this characteristic transmittance. One problem is that there is some overlap in sizes between smoke and dust particles, and therefore the functionality of the filter is usually a compromise between arresting all dust and unintentionally arresting some larger smoke particles thereby decreasing the apparent smoke level seen by the detector, and allowing all smoke through with some dust, thereby increasing the apparent smoke level seen by the detector. Another problem with filters is that they may block over time. If a filter blocks, it may not transmit smoke particles to the detection chamber, thus reducing the effectiveness of the smoke detector. For this reason it is desirable to be able to detect filter blocking before it causes problems in smoke detection.
Any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the invention. It should not be taken as an admission that any of the material formed part of the prior art base or the common general knowledge in the relevant art on or before the priority date of the invention disclosed herein or, any claims defined herein.